1. Field of the Invention
The present invention relates to a current collector substrate in an electrode for use in an alkaline secondary battery, an electrode for use in an alkaline secondary battery using the same, and an alkaline secondary battery having incorporated thereinto the electrode. More particularly, the present invention is concerned with a novel current collector substrate which is advantageous not only in that it can be produced at a low cost, but also in that it has excellent holding ability for a mixture for electrode and exhibits an appropriate current collecting efficiency; a paste-type electrode produced using the above current collector substrate, particularly, a paste-type nickel electrode; and an alkaline secondary battery having incorporated thereinto the above electrode, particularly, a nickel-hydrogen secondary battery.
2. Prior Art
As representative examples of alkaline secondary batteries, there can be mentioned a nickel-cadmium secondary battery and a nickel-hydrogen secondary battery. In the positive electrode (nickel electrode) in the alkaline secondary battery, there are a sinter-type nickel electrode and a paste-type nickel electrode. Of these two types, the paste-type nickel electrode is generally produced as follows.
First, a nickel hydroxide particle which is an active material, a conductive material, for example, cobalt monoxide which forms a conductive matrix during the initial charging after the battery assembly to exhibit a conductive property, a binder, for example, carboxymethyl cellulose, and water are mixed together, thereby to produce a paste of a viscous mixture (hereinafter, referred to as xe2x80x9cmixture for electrodexe2x80x9d) having a predetermined composition.
Then, the thus produced paste of the mixture for electrode is directly coated onto or filled in a current collector substrate, followed by drying. The thickness of the resultant substrate is leveled by, for example, roll-calendering, simultaneously with rendering the dried mixture dense, thereby the mixture is strongly held on or in the current collector substrate.
In this case, as the current collector substrate, generally, a foamed metallic porous material sheet in which fine open pores are three-dimensionally formed is used, and specifically, a foamed Ni (nickel) porous material sheet is used. The reason why such a metallic porous material sheet is used as a current collector substrate resides in that the mixture for electrode which is filled in the open pores distributed in the inner portion of the sheet is in the form of being captured by the open pores, and hence, coming off of the mixture for electrode from the current collector substrate is suppressed, so that the resultant nickel electrode has an excellent current collecting ability.
As mentioned above, when a high capacity battery is produced, as a current collector substrate, the use of the above-mentioned metallic porous material sheet capable of having filled therein a mixture for electrode with a high density is effective. However, the metallic porous material sheet has also the following problems.
The first problem is as follows. For example, the Ni porous material sheet which has been customarily used is generally produced by a process in which a foamed urethane resin is subjected to electroless Ni plating and Ni electroplating successively, thereby to form an Ni plated layer on the skeletal portion of the sheet, and then, the resultant resin is subjected to heat treatment to remove the urethane resin from the skeletal portion by incineration. Due to this process, the price of the sheet becomes high.
When it is intended to produce a battery in a large size capable of performing a large current discharging by increasing the electrode area, the amount of the current collector substrate (Ni porous material sheet) to be used is inevitably increased. Therefore, due to the above problem, the production cost for the battery disadvantageously becomes high.
Another problem is as follows.
Such Ni porous material sheet has poor flexibility. For this reason, in the case where a cylindrical nickel-hydrogen secondary battery is assembled from a nickel electrode produced using the Ni porous material sheet as a current collector substrate and, for example, a negative electrode composed of a metal alloy having hydrogen occluded therein (hydrogen occlusion metal alloy), when the nickel electrode and the hydrogen occlusion metal alloy negative electrode are laminated through a separator and the resultant laminate is spirally wound to form an electrode group, the Ni porous material sheet is damaged by folding and the resultant cracks penetrate the separator and is brought into contact with the negative electrode, causing an occurrence of short-circuiting.
For these reasons, in the recent batteries in a large size capable of performing a large current discharging, it has been started to make a study on a coating-type electrode using a two-dimensional sheet, such as an inexpensive punched metal or expanded metal, as a current collector substrate instead of the metallic porous material sheet, which electrode is produced by coating a mixture for electrode onto the surface of the two-dimensional sheet and drying the coated mixture.
The above-mentioned coating-type electrode can be produced simply by coating a mixture for electrode onto the surface of a two-dimensional sheet at a predetermined thickness and drying the coated mixture. Therefore, the coating-type electrode is easily produced, as compared to the electrode using a metallic porous material sheet. However, this coating-type electrode has also the following problems.
The first problem is as follows. Since the current collector substrate is a two-dimensional sheet, the adhesive strength between the mixture for electrode and the current collector substrate is poor, so that the mixture for electrode is easily released from the surface of the current collector substrate. On the other hand, in the conventional metallic porous material sheet having a three-dimensional network structure, since the mixture for electrode is captured by the open pores in the three-dimensional metallic porous material sheet, coming off of the mixture for electrode does not occur.
Due to such a problem, lowering of the capacity and an increase of the electric resistance of the produced electrode occur, thus leads to lowering of the discharge capacity and lowering of the discharge voltage of the assembled battery. The occurrence of such a problem can be suppressed to some extent by increasing the content of the binder in the mixture for electrode. However, the increased binder adversely affects the active material so as to be lower the reactivity, and it is difficult to achieve a good balance between this problem about the reactivity lowering and the discharge voltage and discharge capacity of the battery.
The second problem is as follows. In the case where the active material in the mixture for electrode is a nickel hydroxide particle, since the nickel hydroxide particle is nonconductive, the electronic conductivity between the nickel hydroxide particle which is present in the position far from the current collector substrate, as viewed in the direction of the thickness of the mixture for electrode layer formed on the surface of the current collector substrate, becomes poor. For this reason, lowering of the utilization of the active material, lowering of the current collecting efficiency, increase in the electric resistance of the electrode, lowering of the discharge voltage, lowering of the discharge capacity and the like occur.
The occurrence of the above-mentioned problems can be suppressed to some extent by increasing the content of the conductive material in the mixture for electrode. However, the conductive materials which have been conventionally used, such as cobalt oxide, cobalt hydroxide and the like, form a conductive matrix during the initial charging after the battery assemble. Therefore, before the initial charging, these materials exhibit no conductivity similarly to the nickel hydroxide particle. For this reason, from the viewpoint of solving the problem about formation of a conductive matrix after the initial charging, the above-mentioned binding property between the mixture for electrode and the current collector substrate and the capturing ability for the mixture for electrode by the current collector substrate serve as extremely important factors, but, when a two-dimensional sheet is used as a current collector substrate, as mentioned above, since the capturing ability is unsatisfactory, as compared to the case of using a three-dimensional metallic porous material sheet, it is difficult to form a matrix having a sufficient conductivity. Therefore, lowering of the capacity as the electrode cannot be avoided.
It is an object of the present invention to provide a current collector substrate in an electrode of an alkaline secondary battery, which is advantageous not only in that it has excellent capturing ability for a mixture for electrode and exhibits an appropriate current collecting efficiency, but also in that it can be produced at a very low cost, as compared to the current collector substrate using a three-dimensional metallic porous material sheet.
It is another object of the present invention to provide a paste application-type electrode for use in an alkaline secondary battery, which is advantageous in that the utilization of the active material in the mixture for electrode is improved.
It is still another object of the present invention to provide an alkaline secondary battery having incorporated thereinto the above-mentioned electrode, which is advantageous in that it is inexpensive and the voltage lowering during large current discharging can be suppressed.
For attaining the above objects, in the present invention, there is provided a current collector substrate for use in an alkaline secondary battery, having a plurality of openings formed within the surface of a metal powder sinter sheet having a thickness of 80 xcexcm or less, wherein each of the openings has a periphery portion comprising a burr portion protruding from the surface of the metal powder sinter sheet.
Specifically, there is provided the current collector substrate, wherein the opening have periphery portions comprising burr portions protruding from the surface of the metal powder sinter sheet in the opposite directions, or wherein the openings which are adjacent to each other have periphery portions comprising burr portions protruding from the surface of the metal powder sinter sheet in the opposite directions, or wherein all of the openings have periphery portions comprising burr portions protruding from the surface of the metal powder sinter sheet in the same direction, or wherein the metal powder sinter sheet is an Ni powder sinter sheet which is produced by a powdery calendering process.
Further, in the present invention, there is provided an electrode for use in an alkaline secondary battery wherein a mixture for electrode is coated onto one surface or both surfaces of the above-mentioned current collector substrate, and preferably, there is provided the electrode for use in an alkaline secondary battery wherein the mixture for electrode contains, as an active material, a nickel hydroxide particle coated with a higher-order oxide of cobalt and/or a nickel hydroxide particle coated with a cobalt compound.
In addition, in the present-invention, there is provided an electrode for use in an alkaline secondary battery containing a nickel hydroxide particle coated with metallic nickel as an active material.
Still further, in the present invention, there is provided an alkaline secondary battery having incorporated thereinto the above-mentioned electrode. Particularly, there is provided a nickel-hydrogen secondary battery wherein the metal powder sinter sheet is an Ni powder sinter sheet, wherein an electrode in which the mixture for electrode contains, as an active material, a nickel hydroxide particle coated with a higher-order oxide of cobalt and/or a nickel hydroxide particle coated with a cobalt compound is used as-a positive electrode, and a hydrogen occlusion metal alloy electrode is used as a negative electrode, as well as a nickel-hydrogen secondary battery using an electrode containing a nickel hydroxide particle coated with metallic nickel as a positive electrode and a hydrogen occlusion metal alloy electrode as a negative electrode.